Skeletal structures are formed of bones and adjoining structures. These skeletal structures may require artificial support or stabilization. For instance, the human spine is composed of a column of thirty-three vertebrae and their adjoining structures. Most of the vertebrae are capable of individual movement and ensure the general movement of the spine: flexion, extension, axial rotation and lateral flexion. An intervertebral disc is positioned between opposing faces of adjacent vertebrae. Each of these vertebrae includes a vertebral body, and a vertebral arch that encompass an opening, called the vertebral foramen, through which the spinal cord and spinal nerves pass. The body of the spinal vertebra is connected to the arch by the pedicles—one on each side of the vertebral arch—, which form two short thick processes.
Unfortunately, there are numerous diseases that deteriorate one or more portion of the vertebra. For example, osteoporosis, scoliosis, kyphosis, spondylolisthesis, tumors, as well as fractures or extreme shocks may leads to the deterioration of vertebrae. As a result vertebrae may collapse, nerves may be pinched causing enormous pain, or regular movement of the column may be limited. In these and other situations, surgical installation of various devices is designed to allow the person to resume a normal life.
One well-known device for spine stabilization procedures is the pedicle screw, which is threaded for engagement with bone. A pedicle screw is surgically installed posteriorly into and through a pedicle. Bone screws, like pedicle screws, are commonly used to fix adjacent bones or bone fragments with respect to each other. For example, bone screws are commonly used to help repair fractures in bone, to attach bone plates to bone, to fix adjacent vertebral bodies, or for stabilizing the spine. When the bone is diseased, for example due to osteoporosis, deteriorated, for example due to prior surgical procedure, or degenerated, securing the bone screw may be problematic. For instance, when a pedicle screw is used on osteoporotic patients, the lack of bone density makes it difficult to properly engage the bone screw threads resulting in a fragile connection between the pedicle screw and bone.
If the threaded portion of the screws does not properly secure to the bone, the bone screw will loosen and pull out or break. This loosening can occur over time. Therefore the use of bone fixation device improving the fixation between the bone and the bone screw, preferably between the vertebra and the pedicle screw, may be needed during surgical procedure.
Operation of an expandable fixation device is often carried in two steps: firstly a bone fixation device is inserted into a hole pre-drilled in the bone and secondly a bone screw, such as a pedicle screw, is inserted into the bone fixation device and ensures expansion of the bone fixation device. With this kind of devices, the bone screw is better secured by insertion into the bone fixation device, and the bone fixation device ensures fixation between the bone and the bone screw.
The bone-fixation devices of the prior art, such as for example the anchor sleeve disclosed in U.S. Pat. No. 4,711,232, are intended to expand radially their distal part upon insertion of a screw. Indeed most of the orthopedic device manufacturers are selling bone-fixation device comprising two slots defining two distal parts which spread apart from each other upon insertion of a screw; the distal parts are anchored in the cancellous vertebral body.
Each vertebra comprises a vertebral body and a vertebral arch. The vertebral body consists of cancellous bone, also named trabecular bone, having high porosity from 30 to 90%. The vertebral arch comprises two hard pedicles consisting mainly of cortical bone, also named compact bone, having low porosity from 5 to 30%.
One of the objects of the invention is therefore to enhance the fixation between the bone fixation device and the vertebra by combining an anchorage inside the soft vertebral body with an anchorage around the hard vertebral pedicle, and optionally an anchorage inside the hard vertebral pedicle. The present invention combines the expansion inside the vertebral body with an anchorage around a pedicle by squeezing the pedicle between a flange and the expanded part of the bone fixation device.
Furthermore, as disclosed herebefore, bone-fixation devices of the prior art, and especially expandable bone-fixation devices, expand radially, especially at their distal end, and thus present wide openings, hollows and asperities, into which bone may grow, preventing the ability to remove the bone-fixation devices.
As bone screws present a thread, they can be removed by unscrewing once the bone has been stabilized. The removal of the bone screw is necessary to avoid leaving unnecessarily foreign body inside the human body. The removal of the bone fixation device securing the bone screw is equally required. However the devices of the prior art with wide openings prevent or severely limit the ability to be removed after insertion.
Another object of the invention is therefore to enhance and facilitate the removal of bone-fixation devices by minimizing the openings and asperities into which bone may grow.
According to the Applicant, the present invention avoids the disadvantages of the prior art by:                combining an anchorage inside the vertebral body with an anchorage around the pedicle and optionally an anchorage inside the pedicle; and        minimizing the openings and asperities and allowing easy removal of the bone fixation device.        